Re: Handling pgoff in perf elf mmap/mmap2 elf info

[Ulf Hermann <ulf dot hermann at qt dot io>]

Consider:

> 0x7fac5ec0b000 to 0x7fac5ed9a000, len =   0x18f000, offset =                0
> r--p    /usr/lib/libstdc++.so.6.0.25
> 0x7fac5ec94000 to 0x7fac5ed8a000, len =    0xf6000, offset =          0x89000
> ---p    /usr/lib/libstdc++.so.6.0.25

0x7fac5ec94000 - 0x89000 = 0x7fac5ec0b000

This is just taking away the 'r' bit from part of the first mapping. We 
can ignore it in perf and perfparser.

> 0x7fac5ec94000 to 0x7fac5ed4c000, len =    0xb8000, offset =          0x89000
> r-xp    /usr/lib/libstdc++.so.6.0.25

Same thing, but adding the 'r' and 'x' bits.

> 0x7fac5ed4c000 to 0x7fac5ed89000, len =    0x3d000, offset =         0x141000
> r--p    /usr/lib/libstdc++.so.6.0.25

0x7fac5ed4c000 - 0x141000 = 0x7fac5ec0b000

This is re-adding the 'r' bit for a different range, again without 
changing the contents.

> 0x7fac5ed8a000 to 0x7fac5ed97000, len =     0xd000, offset =         0x17e000
> rw-p    /usr/lib/libstdc++.so.6.0.25

0x7fac5ed8a000 - 0x17e000 = 0x7fac5ec0c000

Strange, what is this? Note that the 'rw', though. It probably doesn't 
contain any executable code. In effect, perfparser and perf should then 
truncate the original mapping and never report this one to libdw, just 
like we do in perfparser now (for different reasons).

So, what about the following algorithing, which can be done entirely 
outside of elfutils:

If we get an mmap with a pgoff, check if the same file is already mapped 
at the "virtual" start address of the file (start - pgoff), and if it 
is, ignore the mmap. That should deal with the first 3 overlaps here. 
The last one is really mapping a different part of the file, but as the 
mapping is not executable and read-write we should really never get a 
sample from it.

We might actually check the permission bits before reporting things to 
dwfl so that a few broken samples don't destroy the memory map. However, 
that partially contradicts the above. We'd need to OR up all the 
permissions from different mmaps covering the same file at the same base 
address to get an approximation for it, or we need a different data 
structure.

best,
Ulf